Microchannel avalanche photodiode (variants)

ABSTRACT

The invention is directed to an avalanche photodiode containing a substrate and semiconductor layers with various electro-physical properties having common interfaces both between themselves and with the substrate. The avalanche photodiode may be characterized by the presence in the device of at least one matrix consisting of separate solid-state areas with enhanced conductivity surrounded by semiconductor material with the same type of conductivity. The solid-state areas are located between two additional semiconductor layers, which have higher conductivity in comparison to the semiconductor layers with which they have common interfaces. The solid-state areas are generally made of the same material as the semiconductor layers surrounding them but with conductivity type that is opposite with respect to them. The solid-state areas may be made of a semiconductor with a narrow forbidden zone with respect to the semiconductor layers with which they have common interfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/890,750 filed on Feb. 20, 2007, incorporated herein by reference inits entirety for all purposes.

FIELD OF THE INVENTION

The invention relates to semiconductor photosensitive devices andspecifically to semiconductor avalanche photodiodes with internalamplification of the signal. The proposed microchannel avalanchephotodiode can be used for registration of super feeble light pulses,including up to individual photons, and also gamma quants and chargedparticles in devices for medical gamma tomography, radiation monitoring,and nuclear physics experiments.

BACKGROUND OF THE INVENTION

A device is known (Gasanov, A. G. et al., Patent of the RussianFederation No. 1702831 issued Jun. 27, 1997, which includes asemiconductor substrate and a matrix of semiconductor areas that haveconductivity type opposite that of the substrate and that are separatedfrom the translucent field electrode by a buffer—resistive layer withspecific conductivity. Avalanche amplification of the photoelectronstakes place on the boundaries between the substrate and thesemiconductor areas. The avalanche current then flows to the translucentelectrode through the resistive layer situated above these areas. Ashortcoming of this device is the low quantum output in the visible andultraviolet areas of the spectrum due to the poor transparency of boththe buffer layer and the highly doped semiconductor areas. In addition,there is no possibility that the photoelectrons formed between thesemiconductor areas can be amplified, which leads to a reduction in thesensitivity of the device.

A device is known (Antich P. P. et al., U.S. Pat. No. 5,844,291 issuedDec. 1, 1998), which includes a semiconductor substrate with n-typeconductivity and an epitaxial layer with p-type conductivity separatedfrom the substrate by a resistive layer and a dielectric layer. Separatesemiconductor areas with n-type conductivity are formed inside thedielectric layer and exit on the one side at the resistive layer and onthe other—at the epitaxial layer. Highly doped areas with n-typeconductivity ensure localization of the avalanche process in the p-njunctions separated one from the other by areas of the dielectric layer.The photosensitive layer where the photoelectrons are formed is actuallythe epitaxial layer grown on the surface of heterogeneousmaterials—dielectric and resistive layers. That is why the mainshortcomings of the device are the complexity of the technology ofpreparing such epitaxial layers and the high level of dark current,which leads to deterioration of the sensitivity, and the signal-to-noiseratio of the device.

A lastly, a device is also known (Sadygov Z. Y., Patent of Russia No.2102821 issued Jan. 20, 1998), which has been taken as a prototype andwhich includes a semiconductor substrate and a semiconductor layerforming a p-n junction with the substrate. The surface of the substratecontains a matrix of separate semiconductor areas with enhancedconductivity compared to that of the substrate. In the prototype, thesemiconductor areas are used with the purpose of creating separateavalanche areas (micro-channels) that ensure amplification of thesignal. A shortcoming of the device is the presence—and also theformation in the process of operation—of uncontrollable localmicro-sparkovers in the interface regions where amplification of thephotoelectrons is taking place. The problem here is that thesemiconductor areas are located immediately on the p-n junctioninterface formed on the substrate—semiconductor layer interface. That iswhy the semiconductor areas have a charge and current connection betweenthem or through the electrically neutral part of the semiconductor layeror through the substrate depending on the conductivity type. In otherwords, the device does not have implemented local limitation of thecurrent in the separate areas where the avalanche process is takingplace. The one or several areas with decreased sparkover potential donot permit an increase in the device voltage in order to attain a highlevel of the avalanche process over the whole area of the device. Inthis way, the device has a limited factor of amplification of theavalanche process, which is an indicator of the sensitivity level of theavalanche photodiode.

SUMMARY OF THE INVENTION

In brief, the present invention is directed to an avalanche photodiodecontaining a substrate and semiconductor layers with variouselectro-physical properties having common interfaces both betweenthemselves and with the substrate. The avalanche photodiode may becharacterized by the presence in the device of at least one matrixconsisting of separate solid-state areas with enhanced conductivitysurrounded by semiconductor material with the same type of conductivity.The solid-state areas are located between two additional semiconductorlayers, which have higher conductivity in comparison to thesemiconductor layers with which they have common interfaces. Thesolid-state areas are generally made of the same material as thesemiconductor layers surrounding them but with conductivity type that isopposite with respect to them. The solid-state areas may be made of asemiconductor with a narrow forbidden zone with respect to thesemiconductor layers with which they have common interfaces. Thesolid-state areas are made of a metal material.

Stated somewhat differently, the characteristic feature of the proposeddevice is that in the avalanche photodiode containing a substrate andsemiconductor layers with various electro-physical properties havingcommon interfaces both between themselves and with the substrate, atleast one two-dimensional matrix of separate solid-state areas—isletswith enhanced conductivity for the creation of potential micro-holes isformed. In order to reduce the generation current in the volume and toimprove the homogeneity of the distribution of the potential along thesurface of the device, the solid-state areas are situated between twoadditional semiconductor layers having enhanced conductivity withrespect to the semiconductor layers with which they have commoninterfaces. As a result, such form of distribution of the potential inthe volume of the device is achieved, which ensures the gathering of thephotoelectrons towards the separate solid-state areas where avalancheamplification of the charge carriers with their subsequent accumulationin the corresponding micro-holes is taking place. The charge accumulatedin the micro-holes decreases the electric field in the avalanche area,which leads to self-extinguishing of the avalanche process. Then thecharge carriers flow to the contacts, thanks to sufficient leakage inthe micro-holes.

In this way, amplification of the photoelectrons takes place inindependent multiplication channels with subsequent self-extinguishingof the avalanche process. Thanks to that, the stability of the amplitudeof the photo-response is improved and the sensitivity of the photodiodeis increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C represent side cross-sectional views of an avalanchephotodiode in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is aimed at improving the stability of the signalamplitude and increasing the sensitivity of the avalanche photodiode inthe visible and ultraviolet regions of the spectrum. In order to achievethese technical results, at least one matrix consisting of separatesolid-state areas with enhanced conductivity surrounded on all sides bysemiconductor material with one type of conductivity is formed in theavalanche photodiode that includes a semiconductor substrate andsemiconductor layers with various electrophysical parameters. Thesolid-state areas are located between two additional semiconductorlayers, which have enhanced conductivity in comparison to thesemiconductor layers with which they have a common interface. Moreover,at least one of the additional semiconductor layers with enhancedconductivity does not have a common interface with the solid-stateareas. The semiconductor areas are located along the common interface ofthe semiconductor layers.

Depending on the variant of implementation of the device, thesolid-state areas with enhanced conductivity are formed of material,which is the same as that of one of the semiconductor layers but with adifferent type of conductivity, of narrow zone semiconductor with regardto the material of the semiconductor layers, and also of metal material.This leads to the formation in the device of either alternating p-njunctions, or heterojunctions, or metal-semiconductor junctions in adirection perpendicular to the substrate plane.

As a result, at least one two-dimensional matrix of separate potentialholes located between the additional semiconductor layers with enhancedconductivity is formed in the device. The formation of two and morematrices of solid-state areas with enhanced conductivity leads to greatimprovement of the sensitivity and stability of the signal amplitude ofthe device.

The invention is illustrated in FIGS. 1A-C, where the cross sections ofthe micro-channel avalanche photodiode with sample A and sample Cmatrices of the solid states areas located between the additionalsemiconductor layers with enhanced conductivity are shown. The device ismanufactured on a base of a semiconductor substrate 1, for examplesilicon with n-type conductivity and a resistivity of 1 Ωcm. At thebeginning, the first additional semiconductor layer 2 with n-typeconductivity and a resistivity of 0.1 Ωcm is formed in the work area ofthe semiconductor substrate by means of local diffusion doping withphosphorus. Then a silicon semiconductor layer 3 with p-typeconductivity and resistivity in the range of 1-100 Ωcm forming a p-njunction with the first additional semiconductor layer is grown on thesurface of the substrate by means of molecular epitaxy. The solid-stateareas with enhanced conductivity 4 are formed by means of ion-doping ofthe semiconductor layer with phosphorus atoms. The doping dose isselected in the 5-100 μCi/cm² range. After annealing the defects at atemperature of 900° C., areas or islets with n-type conductivity and aresistivity of about 0.01 Ωcm surrounded on all sides by semiconductormaterial with p-type conductivity and resistivity in the 1-100 Ωcm rangeare formed in the semiconductor layer. Then a second additionalsemiconductor layer 5 with a resistivity of about 0.01 Ωcm is formed onthe surface of the semiconductor layer 3 by means of ion-doping withboron. This leads to the formation in the volume of the device ofalternating p-n junctions in direction 6, perpendicular to the substrateplane, whereby the alternating p-n junctions are located between the twoadditional semiconductor layers with enhanced conductivity.

Depending on the variant of implementation of the device, thesolid-state areas with enhanced conductivity are formed also ofgermanium or titanium clusters surrounded by silicon material. In orderthat germanium or titanium clusters can be formed in the volume of thesilicon semiconductor layer, the dose of doping agent with germanium ortitanium is selected to be above 1000 μCi/cm². Then either alternatingp-n junctions, or metal-semiconductor junctions in a directionperpendicular to the substrate plane, are formed in the device.

The cross-section dimensions of the solid-state areas and the clearancebetween them are determined by a special photo-template by means ofwhich windows are opened in a photo-resist or in a special mask forlocal doping of the semiconductor layer. The energy of the ions in theprocess of doping is selected depending on the necessary depth ofimbedding of the implanted atoms. Then the known components of thedevice as the protecting rings or the deep grooves around the work areaas well as the contact electrodes are prepared.

Unlike the prototype, the avalanche amplification of the photocurrent inthe proposed device takes place only in the boundaries of thesolid-state areas with semiconductor layers, which represent bythemselves independent channels of amplification of the charge carrierscoinciding with direction 6. This occurs thanks to the fact that theareas with alternating potential barriers in direction 6 are surroundedby p-n junction areas located in direction 7. In operating mode, avoltage with polarity corresponding to the depletion of thesemiconductor substrate from the main carriers of the charge is added tothe top electrode of the semiconductor layer. When this happens, themiddle junction in the multiplication channel becomes displaced in theforward direction and the two outside junctions—in the oppositedirection. The p-n junction areas located between the multiplicationchannels also become displaced in the opposite direction. Moreover, thefirst additional semiconductor layer with enhanced conductivity limitsspreading of the electric field in the substrate and in this way itensures a decrease in the dark generation current and spreading of theavalanche process only in the work area of the device. The secondadditional semiconductor layer with enhanced conductivity limits theelectric field from the external side and ensures homogeneity of thepotential along the photosensitive surface of the device. As a result, aform of distribution of the potential inside the device is attained suchthat the gathering of the photoelectrons formed in the upperphotosensitive semiconductor layer towards the potential micro-holes isstimulated. The amplification of the photoelectrons takes place in thefirst channel of multiplication from top to bottom, and the nextjunction displaced in the forward direction plays the role of apotential hole with a depth of about 0.5-1 V where the multipliedelectrons gather. The accumulation of electrons in the above-mentionedpotential hole for a time of several nanoseconds leads to a sharpdecrease in the electric field in the avalanche area (i.e., in theboundary area of the first junction) and as a result of this theavalanche process in the given multiplication channel stops. Afterwards,for a time of several tens of nanoseconds after the discontinuation ofthe avalanche process, the accumulated electrons go into the substratethanks to sufficient leakage of the third junction. In this way,avalanche amplification of the photoelectrons takes place in independentmultiplication channels, which do not have a charge connection betweenthemselves. Thanks to that, the stability of operation is improved andthe sensitivity of the avalanche photodiode increases.

What is claimed is:
 1. Avalanche photodiode containing a substrate andsemiconductor layers with various electro-physical properties havingcommon interfaces both between themselves and with the substrate,characterized by the presence in the device of at least one matrixconsisting of separate solid-state areas with enhanced conductivitycompletely surrounded by semiconductor material with one and the sametype of conductivity, whereby the solid state areas are located betweentwo additional semiconductor layers, which have higher conductivity incomparison to the semiconductor layers with which they have commoninterfaces.
 2. Avalanche photodiode in accordance with claim 1,distinguished by the following: the solid-state areas are made of thesame material as the semiconductor layers surrounding them but withconductivity type that is opposite with respect to them.
 3. Avalanchephotodiode in accordance with claim 1, distinguished by the following:the solid-state areas are made of a semiconductor with a narrowforbidden zone with respect to the semiconductor layers with which theyhave common interfaces.
 4. Avalanche photodiode in accordance with claim1, distinguished by the following: the solid-state areas are made of ametal material.